Photoresist is a light-sensitive material commonly used on a semiconductor wafer for creating a pattern on the wafer from a mask design requirement for microchip fabrication. The structure and properties of photoresist change upon exposure to light. For example, in a negative acting photoresist, that portion of the photoresist exposed to light is polymerized to an insoluble condition, whereas that portion not exposed to light remains soluble.
The soluble, unpolymerized photoresist is easily removed to expose the patterned portion of the wafer. In contrast, the polymerized portion remains insoluble and is subsequently hard baked on to the wafer to become an impenetrable etch barrier for the etching process. The etching process permanently transfers the patterned image into the surface layer of the wafer.
After etching, the patterned portion is a permanent part of the wafer, and consequently, the resist layer that has served as an etch barrier is no longer needed. Therefore, the polymerized photoresist must be removed to complete the fabrication of the microchips on the wafer. However, the hard bake has made the photoresist extremely difficult to remove from the wafer. Accordingly, the processes for removing the baked photoresist are commonly known as wet and dry stripping.
Wet stripping generally involves the use of chemicals reacting to remove the hardened photoresist, of which sulfuric acid and hydrogen peroxide are the most common. Although other chemicals are also used, most retain undesirable, and sometimes hazardous features that are commonly associated with chemicals in general.
Dry etching is often referred to as plasma etching. Plasma etching is also a chemical process, but uses plasma energy to force the reaction, thus requiring a chemical etchant and an energy source. The energy sources creates a radiofrequency field through electrodes, consequently energizing a gas to a plasma state which acts as the etch medium. No wet chemicals or rinsing are involved. However, this type of dry etching retains its own undesirable drawbacks since it requires a somewhat complex system to produce the results, i.e., an energy source, chamber, vacuum, and gas supply.
Ion beam etching and reactive ion etching are also dry etching processes, but like unto plasma etching, each retain their own disadvantages and require complex systems necessary to perform the etching.
Obviously, such complex, time consuming, expensive and potentially hazardous methods for removing photoresist from a semiconductor wafer are not desirable features that accompany the prior-an techniques. Accordingly, objects of the present invention are to provide an improved method for removing film from a substrate, such as photoresist from a semiconductor wafer.